Clothing manufacturers are concerned about the danger that needles used for sewing clothes, bags, pouches, etc., if they are left to remain in the products, could prick the human body and to prevent such danger, magnetic inspection for detecting needles has been conducted. Accordingly, nonmagnetic plating, for example, nickel-phosphorus plating or nickel-tin alloy plating has been predominantly used for ornamental articles for clothing. However, in recent years, it has been pointed out that if nickel-containing metal contacts a human body, it can be an allergen to cause skin rashes or inflammation. Several countries in the world, such as European countries and the United States of America, are going to take some measures (legislation) for protecting the human body from such a nickel allergy.
Under the circumstances, copper-tin alloy plating has been reviewed in recent years as promising metal plating that substitutes for nickel alloy plating.
Many techniques have hitherto been proposed for copper-tin alloy plating as disclosed in JP 10-102278 A, JP 2001-295092A (U.S. Pat. No. 6,416,571), etc. However, the conventional techniques have a problem of instability of a disengaging force. That is, when the conventional technique is applied to ornamental articles like snap buttons, which are attached to clothes and repeatedly subject to resilient snap engagement, fluctuation of the disengaging force (i.e. force required for disengaging a snap engagement) becomes greater as engagement and disengagement are repeated, and eventually the disengaging force will be outside of a specific range. As a result, when the disengaging force is too strong, the cloth will be ruptured and on the contrary when the disengaging force is too weak, the button will be disengaged of itself. Note that as shown in FIG. 1, which is a cross-sectional view showing a snap button, the snap button includes snaps used as a set, i.e., a male snap 1 consisting of a stud member 3 having a round head 3a that has a generally extended (flared) top and a fitting member 4 for fitting the stud member 3 to a cloth 7, and a female snap 2 having a socket member 5 resiliently engageable with the round head 3a of the male snap 1 and a fitting member 6 for fitting the socket member 5 to another cloth 8.
Further, when copper-tin alloy plating is applied to clothing ornaments, accessories or the like, the appearance color (color tone) of the plating is considered to be one of the important qualities required. In the copper-tin alloy plating, those platings having a red, yellow (gold), white, or silver white color tone have been realized by varying the contents of copper and of tin in the plating and on the other hand those platings having a black-based color tone have been realized by incorporating cobalt or selenium in the copper-tin plating.
However, since the use amounts of cobalt and selenium in the copper-tin-(cobalt or selenium) alloy in the plating having a black-based color tone are regulated by European Toy Safety Standard EN71-3 or Ecotex Standard 100, copper-tin alloy plating having a black-based color tone without containing any such controlled substances has been demanded.
As far as is known, there has been made only one proposal for the copper-tin alloy plating having a black-based color tone containing no such prohibited substance. That is, JP 10-102278 A discloses a method of producing copper-tin alloy plating having a pale black color tone with a Cu/Sn weight ratio=41/59. The black plating taught in the document has poor disengaging force stability and poor adhesion. As a result, a problem arises, for example, that the plating migrates to the clothes by friction with the clothes, so that the commercial value of the clothes is deteriorated, which prevents commercialization of the above-mentioned pale black copper-tin alloy plating.
Further, industrially operative plating having a black-based color tone for ornamentation and corrosion resistance includes nickel-tin alloy. However, the plating has poor adhesion so that its disengaging force stability is poor and, further, it causes a problem of nickel allergy.